[PPAPI] It is now possible to pass a FileSystem to JS in PostMessage.

Upon receiving a resource, if it is a file system, the V8 Var converter
automatically converts it into a Blink DOMFileSystem and makes it
available in the V8 JavaScript context.

This also adds the possibility of converting other resource types to
JavaScript objects in the future.

BUG=345158

Review URL: https://codereview.chromium.org/173143005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255891 0039d316-1c4b-4281-b951-d872f2087c98
Automated Commit: Committing new LKGM version 5605.0.0 for chromeos.

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255892 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255892